1. Field of the Invention
The present invention relates to an AC generator (or a vehicular alternator) for a vehicle, in particular, relates to a vehicular alternator equipped with a resin end cover (hereinafter, referred to as “an end cover” in short) covering electrical components attached to an outer end surface of a frame.
2. Description of the Related Art
For example, Japanese patent laid open publication No. JP H11-164538 has disclosed an AC generator having a configuration in which an end cover covers a rectifier device to an outer end surface of a frame. The end cover has a cooling window through which cooling air flows from the outside of the AC generator into the inside thereof in order to cool the rectifier device. The rectifier device has cooling fins to which the rectifier elements are fixed and by which the rectifier elements are cooled. Each cooling fin has a plate part and a rib part. The plate part of the cooling fin extends towards approximately a radius direction while facing the end cover. The rib part of the cooling fin protrudes from the plate part towards the end cover. Because the presence of the rib part can increase a thermal discharge area of the cooling fin, and suppress the increase of a temperature-rise at the cooling fin.
A recent trend to decrease a size of an AC generator for a vehicle requires decreasing a gap between the cooling fins of the rectifier device and the end cover which covers the rectifier device, in more detail, requires decreasing a gap between the rib part of the cooling fins and the end cover. In general, because the temperature of the cooling fins becomes high, the temperature of the end cover facing the cooling fins is also increased. Although the end cover made of resin has a large thermal expansion coefficient, because a part in the end cover (hereinafter, referred to as “the rib-part adjacent part in the end cover”), which faces the rib part of the cooling fin projecting toward the end cover, is more close to the cooling fin than another part of the end cover (hereinafter, referred to as “the plate-part adjacent part of the end cover”), which faces the plate part of the cooling fin, the temperature of the rib-part adjacent part of the end cover is higher than that of the plate-part adjacent part of the end cover. As a result, a difference in thermal expansion coefficient between the rib-part adjacent part and the plate-part adjacent part of the end cover generates locally a thermal stress (a stress by a thermal expansion) at a boundary part between each part of the end cover, for example, a boundary part between the rib-part adjacent part in the end cover and the plate-part adjacent part of the end cover. Because such a thermal stress occurs repeatedly every becoming a high temperature of the cooling fins, the durability of the end cover decreases.
Although a separation of the end cover from the cooling fins in the rectifier device can reduce such a problem, the size of the AC generator for a vehicle (or the vehicular generator) increases instead. In addition, although using a thicker end cover can increase its mechanical stress resistance, a total amount of resin is thereby increased. This increases a manufacturing cost.